Squelch circuitry is needed in the receiving section of a high quality RF communication system in order to "squelch" or switch-off what would otherwise be annoying noisy output occuring while the receiver is waiting for a signal to be received. The receiver gain is at its maximum during periods in which no signal is received, and the receiver thus amplifies noise to an unacceptable level during such periods. The basic function of the squelch circuit is to switch-off the output of a receiver when the input signal level is inadequate to provide an intelligible output. This allows a listener to monitor a channel without having to endure long periods of noise output which occurs when no desired signal is being received. When a desired signal is received, its level at the receiver input must be greater than a preset squelch threshold, in order for the squelch circuitry to enable the receiver.
In certain relay link circuits, the squelch circuitry is used to enable a transmitter to retransmit the received signal. For this application, the squelch threshold is set high enough to prevent unwanted triggering of the transmitter from undesired sources such as background signal or noise. The choice of the proper level for the squelch threshold is, however, a compromise. As the threshold is set higher, the receiver range is reduced. Therefore, the optimum choice is that level just above the background signals and noise. If the threshold does not remain constant as the temperature changes, the threshold must be set so that no unwanted trigger occurs even at the worst case temperature. This, however, renders the system less sensitive than it might have been at other temperatures. Minimizing temperature dependence of the squelch trigger is therefore an important goal in the design of an RF receiver or relay link.
The squelch circuitry in a receiver essentially provides an accurate measure of the level of the signal being received. This input signal level can be as low as -107 dBm in some applications, thus requiring approximately 107 dB of gain in order to provide a measurement. This large amount of gain is difficult to stabilize with temperature. It would therefore be a significant advance to provide a method for accurately measuring the receiver's input signal level, without directly relying on the temperature stability of the receiver's gain.
In general, prior squelch control circuits can be divided into a first type that measures signal level at the input of the receiver, and a second type that measures the signal-to-noise ratio at the input of the receiver. In the first type of circuit, the control voltage within an AGC loop may be used as a measure of the input signal level. The biggest problem facing this type of squelch system is accurate measurement of the input signal level. For example, if a squelch circuit were set to trigger on an input signal level of -107 dBm, then 107 dB of stable gain would be required to amplify the signal to the point where it can trigger a comparator. If the trigger point is to remain constant, then all 107 dB of gain must remain constant. This is extremely difficult, because even with temperature compensation, that much gain would vary at least 10 dB over a typical temperature range. Another problem facing the first type of system is that they can be triggered with a pure noise input, if the noise input power reaches the appropriate level. This means that squelch trigger points in such systems must be set high enough so that they do not trigger on even the highest expected input noise level. This, of course, means that the receiver cannot be more sensitive even when the input noise level is low.
The second type of system, to measure signal to noise ratio at the receiver input, must open up the receiver's bandwidth to obtain a sample of "noise only." The wider receiver bandwidth allows extra noise into the circuit all the way to the demodulator. After demodulation, the signal is separated from the extra noise by filters, and a measurement of relative level is made. However, the receiver bandwidth must be wider than the information bandwidth in this type of system, thus rendering the receiver more susceptible to interference. In particular, any signal that appears in the noise measurement band will deactivate the receiver even if a desired signal is also present, unless the desired signal level is significantly greater than the undesired signal level.